1. Field of the Invention
This invention relates to a carrier for a waste gas-treating catalyst, a method for the production of the carrier, and a waste gas-treating catalyst incorporating the carrier therein. More particularly, it relates to a thermally stable and chemically stable carrier for a waste gas-treating catalyst, a method for the production of the carrier, and a catalyst incorporating the carrier therein and used for effecting complete oxidation of a waste gas containing such combustible gases as noxious and odorous organic compounds and carbon monoxide and defecting the waste gas of such harmful substances, a catalyst for cleaning a waste gas emanating from an internal combustion engine, or a catalyst for removing nitrogen oxides (hereinafter occasionally referred to as "NO.sub.x ") from a waste gas emanating from a varying industrial process using a boiler, a gas turbine, or a heating furnace by causing catalytic reaction of ammonia upon the nitrogen oxides.
2. Description of the Prior Art
In recent years, organic solvent type waste gases emanating such as from printing, coating, and resin processing plants, odorous waste gases arising from refuse incinerating furnaces, scrap preheating furnaces, etc., and organic plant waste gases issuing from petrochemical plants have been causing a hygienic problem from the environmental point of view. The purification of these waste gases, therefore, constitutes an urgent task for the prevention of environmental pollution.
As means of purifying these waste gases, the method of direct combustion, the method of adsorption with activated carbon, and the method of catalytic oxidation, for example, have been practiced heretofore in the case of organic solvent type waste gases.
In the method resorting to the catalytic oxidation, the catalyst to be employed uses a carrier made of activated alumina or cordierite.
The method resorting to direct combustion, for example, has the possibility of inducing secondary environmental pollution as by secondary production of nitrogen oxides in addition to necessitating heating at an elevated temperature and incurring high cost of fuel. The method resorting to adsorption entails complicate operations in regeneration, replacement, etc. of the adsorbent and fails to fit the treatment of a waste gas of high concentration in terms of running cost. In contrast, the method resorting to catalytic oxidation is characterized by effecting combustion and decomposition of organic components on a catalyst at a relatively low temperature and enjoying a low cost of fuel and only sparing susceptibility to secondary production of nitrogen oxides. Moreover, this method can be expected to enjoy recovery of the heat owing to the combustion of combustible substances in the waste gas.
The conventional carrier made of activated alumina or cordierite, however, has a weak point that it has the strength thereof degraded by sulfur oxides. Further, the activated has a problem that it is deficient in thermal stability and the cordierite has a disadvantage that the catalyst must be applied in the form of a coating to the surface of a carrier because this substance is deficient in specific surface area. One catalyst uses a carrier of a titania type composite oxide for the purpose of enhancing the durability to resist acidic substances. It is not sufficient satisfactory, however, from the standpoint of thermal resistance such as irreversible thermal shrinkage.
The methods for the removal of nitrogen oxides from waste gases are broadly classified under the method of adsorption, the method of absorption, and the method of catalytic reduction. Among other methods mentioned above, the method of catalytic reduction abounds in ability to treat waste gas, has no use for the otherwise inevitable treatment of waste water, and proves to be advantageous technically and economically.
The method of catalytic reduction is available in two types, i.e. the method of non-selective reduction using a hydrocarbon such as methane or LPG, hydrogen, or carbon monoxide as a reducing agent and the method of selective reduction using ammonia as a reducing agent. The latter method proves to be a very advantageous way of removing nitrogen oxides because it is capable of selectively removing nitrogen oxides from a waste gas containing oxygen in a high concentration and is economical in respect that the consumption of the reducing agent in the operation is small.
The catalysts of the class used in the largest amount in commercial operations have carriers of titanium oxide. The catalyst formed of an oxide of the combination of titanium with vanadium, tungsten, molybdenum, and iron or the catalyst formed of the oxide partly incorporating therein a sulfate, for example, not only keeps its activity in the reduction of nitrogen oxides unaffected by SO.sub.x, oxygen, carbon dioxide, and steam which coexist in the waste gas but also exhibits a desirable activity as compared with the catalyst having activated alumina as a substrate and excels also in acidproofness. This catalyst, therefore, predominates among the catalysts which are used in the method of selective catalytic reduction using ammonia as a reducing agent.
The catalysts which use carriers of titanium oxide as mentioned above, however, are deficient in resistance to heat. The inventors have experimentally confirmed that when this catalyst is continuously used at temperatures in the range of 300.degree. to 400.degree. C., the carrier thereof suffers from a decrease of its specific surface area and sustains crystallization of its composition. This catalyst, therefore, has a disadvantage that the catalytic activity thereof is degraded by use. The catalyst to be used for the removal of nitrogen oxides from the waste gas of the kind under discussion is required to possess a quality such that the resistance thereof to heat excels that of the conventional catalyst.
The nitrogen oxide-containing waste gas emanating from a gas turbine, a steel making open-hearth furnace, or a glass melting furnace, for example, has high temperatures in the range of 500.degree. to 600.degree. C. The waste gas emanating from an ordinary boiler under certain conditions has high temperatures exceeding the level of 400.degree. C.
An object of this invention, therefore, is to provide a novel carrier for use in a catalyst for the treatment of a waste gas, a method for the production of the carrier, and a waste gas-treating catalyst incorporating the carrier.
Another object of this invention is to provide a carrier for a waste gas-treating catalyst which possesses thermal stability and chemical stability, the qualities indispensable for the carrier in withstanding the impacts of the waste gas treatment, a method for the production of the carrier, and a catalyst incorporating therein the carrier and effecting complete oxidation of a waste gas containing noxious or odorous organic compounds and such combustible compounds as carbon monoxide and attaining purification of the waste gas by removal of such harmful substances therefrom.
Yet another object of this invention is to provide a carrier exhibiting high resistance to heat, accomplishing effective removal of nitrogen oxides at working temperatures in a wide range as compared with the conventional countertype, and retaining physical and chemical stability intact for a long time and a catalyst incorporating the carrier therein.